When glass is heated by means of an oscillating roller furnace, the problem is that the edges of the glass curve upwards at the initial stage of the heating. This is due to the great heat flow onto the lower surface of the glass caused by the ceramic rollers used in the furnace at the initial stage of the heating cycle, compared with the heat flow onto the upper surface of the glass. Consequently, the edges of the glass curve upwards, the central area of the glass being easily affected by optical errors, and in addition, the glass becomes unevenly heated. When selective glasses are heated, the situation is particularly difficult, because selective glasses reflect heat radiation particularly intensively. Glasses with selective surfaces are usually heated in such a way that the selective surface faces upwards, whereby the heating of the upper surface of the glass is remarkably more difficult compared with the heating of the lower surface of the glass. Thus, the heating times of selective glasses are naturally longer than the heating times of ordinary clear glass, whereby the capacity of the furnace is typically rather low when selective glasses are heated.
FI patent 62043 discloses a method of preventing the curving of glass. In this method, the upper surface of the glass is subjected to a heat flow by means of forced convection, which heat flow compensates for the heat flow from the rollers below. The forced convection has been provided by blowing horizontal narrow air jets in the longitudinal direction of the furnace, which provide a turbulence effect of air with an injector effect onto the upper surface of the glass. The air jets have been achieved by taking pressurized air compressed by compressors from a compressed-air network outside the furnace. FI patent 83072 discloses a corresponding method, in which the air blown as air jets is also circulated through the lower section of the furnace, whereby the air is heated during this extra round. At the same time, the heat transferred to the air is taken from the lower part of the glass. In both of the methods the effect of the convection is rather low, whereby the method is rather ineffective. The air to be conveyed into the furnace is cold, so that it cools the furnace in its entirety, which increases the energy consumption of the furnace in total. Further, a problem is the uncontrolled discharge of the air to be blown from the furnace. Further still, the emphasis in the method is on intensifying the heating of the upper surface of the glass at the initial stage of the heating. Thus, the total heating time of selective glasses is long, because selective glasses are, in any case, primarily heated from the lower part of the glass applying the radiation principle.
EP publication 0897896 discloses a solution in which coated glass is heated by blowing air onto it from longitudinal blowpipes. The air to be blown is taken from a compressed-air network outside the furnace. The arrangement comprises a compressed-air tank which is provided with overpressure by means of a compressed-air compressor. Due to the compressed-air arrangement, the structure of the solution becomes complex and expensive. If cold air is blown into the furnace, it cools the furnace in its entirety, and thermal energy has to be directed into the furnace in some other way. Heating the air to be blown, in turn, requires a large amount of energy and capacity, so that what it comes to the energy economy, the solution according to EP publication 0897896 is, as a whole, poor. Further, the uncontrolled discharge of the air to be blown from the furnace is a great problem.
FI publication 962158 discloses a method in which the surfaces at the lower side of the glass are cooled at the initial stage of the heating cycle, and correspondingly, the heat transfer of the lower side is intensified at the final stage of the heating cycle by blowing hot air directly to the lower surface of the glass. FI publication 962162 discloses a solution, in which the heating resistors are dimensioned and their control implemented in such a way that the heating resistors are many times more efficient, whereby the heating of the glass at the initial stage of the heating cycle may be performed by utilizing the upper resistors only. The methods are very efficient and well-functioning, but it would be desirable, particularly when heating selective glasses, to make the heating time shorter.
A known solution is also what is known as a convection furnace, in which the intention is to heat glass by blowing hot air onto the upper and lower surfaces of the glass, as well as to the ceramic rollers. In such a solution, air is circulated in the furnace with blowers constructed inside the furnace, whereby the flow velocity of the air is increased, the aim being thus to increase the effect of the air on the surface of the glass. The air is blown at a pressure of approximately 0.005-0.01 bar. The air is heated in the solution either prior to the blower or after the blower. A problem of the solution is particularly the high manufacturing cost, and the slow speed of the heating due to the large mass of the air channels constructed inside the furnace, and uncontrollable heat expansions of the construction.
U.S. Pat. No. 4,505,671 discloses a solution in which glass is heated by blowing heated gas onto its upper and lower surfaces. The gas is taken from a separate gas source and heated with a separate heater. The solution consumes considerable amounts of gas from the gas source. Further, heating gas consumes energy. Increasing the amount of flowing gas and thus increasing the heat-transfer coefficient is in this solution rather difficult.
U.S. Pat. No. 4,059,426 describes a solution in which the glass is supported by gas jets and air is blown with a blower onto the surface of the glass sheet, the air being circulated back to the blower. However, this kind of solution does not allow the surface of the glasses to be subjected to a sufficient heat effect. Further, the publication discloses a solution providing air circulation inside a furnace by utilizing the Coanda phenomenon. The air circulation inside a furnace does not provide a sufficient heat effect on the surface of the glasses either.
Moreover, a solution is known in which the glass is heated in two steps. At the first stage, a lower temperature is used, whereby air having a temperature of about 300 to 400° C. is circulated in the furnace by means of blowers. The air is blown directly onto the upper and lower surfaces of the glass and heated prior to the blowers. At the latter stage, the glass is heated using mainly radiation heating. In this solution, too, the problem has turned out to be the high cost of the air channel system constructed inside the furnace and of the blowers used in the solution. Further, the heating of the glass at the latter stage takes rather a long time, particularly when selective glasses are heated.
An object of this invention is to provide an improved method and apparatus for heating glass.